A U.N. peacekeeper from Cambodia was shot dead during an ambush in the Central African Republic on Tuesday and Prime Minister Hun Sen has called for the safe release of three others who went missing.

“I request the United Nations—who is responsible for this—to pay attention to ensure the lives of Cambodian soldiers,” Mr. Hun Sen said.

Im Sam, in a photograph posted to Hun Manet’s Facebook page.

The National Center for Peacekeeping Forces, Mines and ERW Clearance (NPMEC) said the attackers on Monday were attempting to free imprisoned comrades who had been detained by Moroccan forces working alongside Cambodian troops.

“We are sorry and I take this opportunity to offer condolences to our forces in Central Africa, who were…attacked by rebels, causing one death, and three others have gone missing,” Mr. Hun Sen said during a speech in Tbong Khmum province.

“The goal of their attack is to liberate captives and it’s located near the place where our forces had stayed, so if I can estimate, I think that they may require an exchange between their captives with our soldiers,” he said.

The announcement was followed by a Facebook post by Long Dimanche, ambassador to South Korea, who wrote that a captain had been beheaded by rebels.

“I would like to inform [you] of shocking and sad news of a Cambodian peacekeeper who lost his life in Central Africa in a peacekeeping mission in the framework of the United Nations,” he wrote. “Rebels have beheaded him.”

The post had been edited by the afternoon, omitting the sentence relating to the alleged beheading.

A statement released by the NPMEC in the afternoon named the slain man as Im Sam, a bulldozer driver, and the three missing soldiers as warrant officer Seang Norint, assistant warrant officer Mom Tola, and Mao Eng, who was part of the medical staff. Thuch Thim, a driver, sustained injuries to his left arm, it added.

A peacekeeper salutes during a ceremony at the Kompong Cham Military Air Base in October 2014, ahead of a yearlong mission to the Central African Republic. (Siv Channa/The Cambodia Daily)

The attack took place at about 8 p.m. on Monday near the southeastern city of Bangassou as the 12-man Cambodian engineering unit was returning from a mission in the Rafai area while being protected by Moroccan troops, it said.

“There was ambush fire on the force convoy of MINUSCA by an unidentified armed group,” it said, in reference to the U.N. peacekeeping mission based in the Central African Republic.

“Please note also that this attack is the rebel forces attacking to liberate the prisoners brought by Moroccan forces,” it said.

MINUSCA forces were scouring the area by foot and helicopter in an attempt to find the missing troops, it said.

Five Moroccans were also injured in the attack, and one was missing, it added.

Contacted in the evening, Malinda Kosal, spokeswoman for the U.N.’s National Center for Peacekeeping Forces, said that Im Sam had been shot in an “unidentified ambush” and the body had already been sent to the capital, Bangui.

A statement released by MINUSCA last night condemned the attack and said it would “do everything possible to ensure that the perpetrators of the attack are arrested so that they can be brought to justice.” It also noted that harming the life of a peacekeeper could be considered a war crime.

Parfait Onanga-Anyanga, the head of MINUSCA, extended his “deepest and heartfelt condolences to the family of the victim, his contingent and his country” in the statement.

Despite claims he was gunned down, a photograph purportedly of Im Sam’s corpse, posted to Facebook by Interior Ministry official Pheng Vannak, showed huge gashes across his stomach.

Violence erupted in the Central African Republic in 2013, when Muslim rebels toppled the Christian president. The U.N. began a peacekeeping mission in 2014 and now has more than 12,000 troops in the country to safeguard civilians from eruptions of violence between Christian and Muslim factions.

The U.N. said over the weekend that four international aid groups would temporarily withdraw their workers from areas in the north of the country due to a surge in attacks against them.

Since 2006, Cambodia has sent more than 4,180 personnel on U.N. peacekeeping missions in Sudan, South Sudan, Central African Republic, Chad, Syria, Lebanon, Mali and Cyprus.

In January, a military officer became the fifth Cambodian peacekeeper to die on a U.N. mission after he succumbed to complications from malaria in the Central African Republic. The four other deaths occurred in Mali—two from food poisoning, one in a violent sandstorm and a fourth from malaria.